This invention relates to a power supply circuit comprising a cascade arrangement of a full-wave rectifier and a first and a second switched voltage converter, the first switched voltage converter comprising a first inductive element, first switching means, a first rectifier diode and a first storage capacitor and input terminals which are coupled to output terminals of the mains rectifier for receiving a rectified mains voltage. The second switched voltage converter comprises a second inductive element, second switching means, a second rectifier diode and a second storage capacitor and input terminals which are connected to the first storage capacitor, said second voltage converter being coupled to output terminals of the power supply circuit for supplying an output voltage. A control circuit is provided for controlling the period of conductance of the switching means of the switched voltage converters with a switching period which is much smaller than the cycle period of the mains voltage.
Instead of using two cascade-arranged voltage converters, it is common practice to use one voltage converter with a storage capacitor between the mains rectifier and the voltage converter. A voltage ripple is then produced across this storage capacitor and this voltage ripple is corrected by means of the converter. Moreover, the output voltage is maintained at a constant value, independent of load variations and/or mains voltage variations.
A drawback of this construction is that the input storage capacitor must have a large value if the above-mentioned circuit is to operate satisfactorily. Consequently, the current taken from the mains is peak-shaped and discontinuous, resulting in strong disturbances of the mains supply (mains interference). Mains interference is prevented by arranging a further switched voltage converter, which is commonly referred to as a preconditioner, between the mains rectifier and the switched voltage converter. Beginning about 1996 the requirements for the prevention of mains disturbances will become stricter, which will also apply to display devices, computers and the like. In about 1996 the European standard EN 60555 based on IEC-555 will take effect. The first switched voltage converter (preconditioner) is then used to improve the duty cycle and to reduce harmonics of the current taken from the mains, while the second switched voltage converter is used to correct the mains ripple left on the storage capacitor of the first switched voltage converter.
The power supply circuit described in the opening paragraph is known from German Patent Application DE-3328723. In this solution an up-converter is used as a first converter. The object of arranging an up-converter between the mains rectifier and the existing converter is to take a continuous current from the mains. Although the current taken from the mains is continuous in this solution, the current is peak-shaped so that the mains is still polluted with higher harmonics. Moreover, this circuit also comprises a third switched converter between the second switched converter and the load, making this solution complicated and expensive.
Another solution is described in German Patent Application DE-4032199. In this Application a down-converter is used as the first converter. In this solution the current which is taken from the mains is mainly sinusoidal but discontinuous. This is due to the fact that the down-converter can only take a current from the mains when the rectified voltage exceeds the voltage across the storage capacitor of the down-converter.